A weight for vibration motors has conventionally been formed of tungsten which has a large specific gravity so as to increase a vibration amount.
When a weight made only of tungsten is attached to a shaft (a rotating shaft) of a motor, the shaft needs to be press-fitted in a through-hole of the weight as disclosed in JP-A-2006-345599 (counterpart U.S. publication is: US 2006/0284501 A1), or alternatively, an opening provided in the weight needs to be crimped so that the weight is secured to the shaft as disclosed in JP-A-10-336949.
In the case when the shaft is press-fitted in the through-hole, a powerful and massive press fitting apparatus is used thus requiring a large-scale manufacturing system, and the number of manufacturing steps increases.
On the other hand, in the case when the opening is crimped to the shaft, the shaft is secured by the deformation of the crimped portion, wherein the fastening force at the crimped portion may decrease over time due to deterioration with age while the crimped portion receives stress and vibration during the motor rotation.
Furthermore, since the conventional vibration motor weight is made of a uniform material having a high specific gravity, the rotational moment is inevitably large which is unfavorable to the start-up characteristics. Additionally, since the conventional weight has a large mass, the starting current of the motor is increased to such an extent as to cause a problem. Further, since the weight is wholly made of tungsten, which is expensive, the production cost is increased.
In order to solve these problems, the weight of a vibration motor weight is reduced, the rotation performance of a vibration motor is enhanced. In addition, to reduce the production costs, the vibration motor weight is secured to a shaft of the vibration motor with a resin. An example of such configuration is disclosed in JP-A-9-182363.
FIGS. 8A-8E are diagrams showing some configurations of vibration eccentric weights disclosed in the publication JP-A-9-182363.
As viewed in cross section, an eccentric weight 100 shown in FIG. 8A is made of a substantially trapezoidal weight part 101 and a resin part 102 in which a shaft holding portion 102b is provided on a semi-circular portion 102a which grips the weight part 101 in such a manner as to project therefrom.
As viewed in cross section, in the substantially trapezoidal weight part 101, a top side 101a of the trapezoid configures a continuous arc together with an outer circumferential side 102c of the resin part. A horizontal distance or width A between lateral sides 101b of the weight part 101 is moderately narrowed or decreased as they extend towards the top side 101a. 
As viewed in cross section, while the resin part 102 is configured to grip the lateral sides 101b the horizontal width between which is moderately decreased as they extend towards the top side 101a of the weight part 101, since the horizontal width A is made to be decreased moderately, the holding force of the resin part 102 with which the weight part 101 is held thereby is weak and is more or less a force which can resist a centrifugal force generated when the weight part 101 rotates.
Accordingly, a structural problem resides in the resin part 102 that in the event that an excessive impact which exceeds an anticipated centrifugal force is applied to the resin part 102, the resin part 102 is forcibly expanded to such an extent that it fails by the weight part 101.
As viewed in cross section, an eccentric weight 110 shown in FIG. 8B is made of a substantially arc-shaped weight part 111 having a constant width B and a resin part 112 in which a shaft holding portion 112b is provided on a semi-circular portion 112a which grips the weight part 111 in such a manner as to project therefrom.
As viewed in cross section, in the weight part 111, an outer arc-shaped side 111a of the weight part 111 configures a continuous arc together with a circumferential side 112c of the resin part 112. A horizontal width C between lateral sides 111b of the weight part 111 is gradually increased as they extend towards the outer arc-shaped side 111a. 
As viewed in cross section, while the resin part 112 is configured in such a manner as to grip the lateral sides the horizontal width C between which is made to be increased gradually as they extend towards the outer arc-shaped side 111a of the weight part 111, since the horizontal width C is made to be increased gradually towards the outer arc-shaped side 111a, the holding force of the resin part 112 with which the weight part 111 is held thereby is extremely weak. Because of this, the weight part 111 is held on to the resin part 112 by virtue of the adhesion of an adhesive, and the holding force of the resin part 112 with which the weight part 111 is held thereby is more or less a force which can resist a centrifugal force generated when the weight part 111 rotates.
Accordingly, a structural problem resides in the resin part 112 that in the event that an excessive impact which exceeds an anticipated centrifugal force is applied to the resin part 112, the resin part 112 becomes unable to hold the weight part 111.
As viewed in cross section, an eccentric weight 120 shown in FIG. 8C is made of a semi-circular arc-shaped weight part 121 having a constant width D and a resin part 122 in which a shaft holding portion 122b is provided on a semi-circular portion 122a which holds the weight part 121 on an outer side thereof with an adhesive in such a manner as to project therefrom. As viewed in cross section, the resin part 122 is configured to hold the weight part 121 on the outer side thereof with the adhesive. Because of this, the weight part 121 is held on to the resin part 122 by virtue of the adhesion of the adhesive, and the holding force of the resin main body 122 with which the weight part is held thereby is more or less a force which can resist a centrifugal force generated when the weight part 121 rotates.
Accordingly, a structural problem resides in the resin part 122 that in the event that an excessive impact which exceeds an anticipated centrifugal force is applied to the resin part 122, it is anticipated that the resin part 122 becomes unable to hold the weight part 121.
An eccentric weight 130 shown in FIG. 8D illustrates an example in which the weight part of the eccentric weight shown in FIG. 8A is divided into two weight parts 131a, 131b of a smaller size, and this involves the same structural problem as that of the eccentric weight of FIG. 8A.
An eccentric weight 140 shown in FIG. 8E illustrates an example of a resin part 142 in which a side confronting a semi-circular arc-shaped side is made of straight lines E which are connected to each other in such a manner that a shaft holding portion 141 does not project therefrom, and this involves the same structural problem as that of the eccentric weight of FIG. 8A.
Thus, it is said that in the related-art eccentric weights shown in FIGS. 8A-8E, the resin part has the holding force which is more or less the force which can resist the centrifugal force which is applied to the weight part in the radial direction when the eccentric weight rotates in cooperation with the adhesive.
In particular, when viewing only the configurations of the eccentric weights themselves without considering the adhesion of the adhesive, in the examples shown in FIG. 8A, the configuration in which the resin part holds the weight part relies only on the wedge configuration which tapers with a slight inclination, and hence, the resin part cannot hold the weight part strongly and rigidly. When looking at the examples shown in FIGS. 8B and 8C from the same viewpoint, no configuration for holding the weight part is adopted.
The related art examples have the problem that they adopt the configurations having the extremely weak holding force or no holding force in consideration of the centrifugal force applied to the weight part.
Further, in recent years, there is emerging a demand to resist an unexpected strong impact such as impact resulting from the fall of a vibration motor. However, it can be said when looking at their configurations that the related art examples have a problem that they cannot resist such an impact.